1. Field of the Invention
This invention relates to the field of sonar, and more particularly to the field of demultiplexer circuits used in demultiplexing the multiplexed signal information obtained from DIFAR sonobuoys. This invention provides a test circuit for testing a DIFAR DEMULTIPLEXER CIRCUIT.
DIFAR sonobuoys are dropped in numbers by intelligence gathering aircraft into ocean areas that are to be subjected to short term sonar monitoring. As shown in FIGS. 1, the DIFAR sonobuoys typically have a first dipole directional sonar hydrophone that respond to sound along a sonobuoy (Y) axis, a second dipole directional hydrophone that respond to sound along a sonobuoy (X) axis, and a non-directional or omni hydrophone that responds to sound with uniform sensitivity on all level radials from the instrument.
The DIFAR sonobuoy is typically characterized to float at a predetermined depth for a predetermined duration. The (X) and (Y) axes are orthogonal and are kept float level by operation of the hydrostatic design of the sonobuoy. The sonobuoy is equipped with a fluxgate compass. No means is provided to orientate the sonobuoy once it is in the water.
Sounds received by the hydrophones in the sonobuoy are multiplexed in accordance with a predetermined DIFAR format for transmission to an aircraft sent to record the received data.
The DIFAR format requires that the following conventions be adopted. Using a top view of the sonobuoy, angle Phi is measured clockwise from magnetic north to the +Y axis. Angle Theta is measured clockwise from the +Y axis to the target location. When dropped into the ocean, the angle phi can be any angle from 0 to 360 degrees. The sonobuoy orientation can continually change due to wave action. The relative amplitude and polarity of target signals from the Y axis hydrophones varies as the cosine of angle theta. The relative amplitude and polarity of target signals from the X axis hydrophones varies as the Sine of angle theta. The omni hydrophone signal is positive in all quadrants of angle theta.
The sonobuoy circuitry provides a 15.0 kilohertz signal with four (4) outputs at 90 degree phase intervals. The first signal referred to as the 15.0 kilohertz phase reference signal is phase shifted or delayed in time by an interval proportional to the magnetic heading angle phi to generate a signal referred to as the phase pilot signal.
The Y axis hydrophone information is referred to as cosine channel information. The X axis channel information is referred to as sine channel information. The cosine channel information is multiplied by a cosine modulator signal having a frequency of 15.0 kilohertz. The cosine modulator signal leads the 15.0 kilohertz phase reference by 90 degrees. All delays are measured with respect to the phase reference signal. The sine channel information is multiplied by a sine modulator signal having a frequency of 15.0 kilohertz and is phased shifted to lead the cosine modulator signal by 90 degrees.
The sonobuoy also provides a constant amplitude 7.5 Kilohertz frequency pilot signal. The frequency pilot signal is by definition exactly one half the frequency of the phase reference signal. The phase relationship between the frequency pilot signal and the phase reference signal is not defined.
The omni signal is not multiplied by a carrier. The bandpass of the omni signal information is typically under 2.5 kilohertz. An RF carrier is modulated with the sum of the frequency pilot, the phase pilot, the cosine channel, sine channel and the omni signal information and transmitted by the DIFAR sonobuoy to the sampling aircraft to be recorded on board the aircraft or for retransmittal to a receiving station as a single channel signal for demultiplexing by the DIFAR demultiplexing circuit.
2. Prior Art
Previous DIFAR demultiplexer test circuits did not provide the capability to select a variable heading angle and target angle to test all functions of the DIFAR DEMULTIPLEXER CIRCUIT. The invention DIFAR demultiplexer test circuit provides the required accuracy and flexibility to test and adjust the DIFAR demultiplexer circuit under all conditions. This application relates to an application titled DIFAR DEMULTIPLEXER CIRCUIT Ser. No. 164050, filed 03/04/88, having common inventorship and assignee.